I have a Canon t6i and recently the pictures I have taken in manual mode are coming out black. When I use live shooting the screen is black. However there is a brief moment when the camera 'focuses' that I can see what Im taking a picture of, but the pictures still come out black. I have tried changing the aperture and shutter speed but it didn't help. Ive also checked if the shutter was stuck. All other modes seem to be working fine.
I have tried changing the aperture and shutter speed but it didn't help.
Not to state the obvious, but what you change the aperture and shutter speeds to matters a lot. What were the settings you used? Also, check the ISO and exposure compensation settings.
A good way start is to take a photo in Program mode. If the exposure looks good, check the values that the camera chooses for aperture, shutter speed, and ISO. Switch to manual and use the same settings that the camera did, and you should get a similar exposure (assuming the lighting hasn't changed).
Also, learn to use the light meter. In manual mode, the meter will show you how much light the sensor will see given the current exposure settings.
When you're in manual mode, you can shoot yourself in the foot all you want. This includes underexposing to the point of having a black frame. Check your meter. Unlike all the other automated modes, in M mode, the camera doesn't automatically adjust your settings to shove your meter's "needle" to 0. This is actually a feature, not a bug. But if you haven't shot in M mode before, you may be so used to the camera always putting the needle on 0 for you that you no longer look at it, or think you need to do anything like that.
"0", btw, isn't always exactly correct, it's just what your camera's autoexposure system thinks is correct. Which is why we like having M mode: for those times the AE system is wrong and the meter is misleading.
Do not take this wrong, but all the other modes are automatic. This one depends on you, so... learn how to control the camera.
The situation you describe sounds to me that it is very dark, because you say that the camera focus for a moment and that is the only moment you see what are you photographing. That can be the focus asistant light. The rest of the time the scene is black.
Try to learn photography in daylight not in the dark.
Find this indicator on your camera:
It tells you the light is ok when it is on the middle. Read your camera manual.